Chemically functional nanoscale devices such as sensors are typically built up in a series of steps. Such steps may include writing a chemical or protein ink onto a substrate to provide a new or different functionality, electro-chemical modification of an already deposited substance, or manipulation of a nanoscale object or material. These steps are often performed in sequence with a need to modify the characteristics of, or change or replace, the scanning probe that is being used, before proceeding on to the next step. After such modification or change to the scanning probe, it must be repositioned with sufficient accuracy on the substrate to allow the next step in the process to be performed.
One example of such a patterning process is the use of Dip-Pen Nanolithography (DPN) to fabricate sensor arrays or molecular circuits. Dip-Pen Nanolithography (DPN) is a direct-write printing technique that uses chemically modified AFM tips to pattern biological materials on a variety of substrates at the nanoscale level. DPN is discussed in detail by Ginger et al. in “The Evolution of Dip-Pen Nanolithography”, Angew. Chem. Int. Ed. 43, 30-45 (2004), which is incorporated by reference herein for all purposes.
There are numerous examples of chemical and biological materials that can be patterned at resolution better than 100 nm, and in the best case, as high as 15 nm, a length scale that is on the order of the dimensions of individual biological macromolecules. For example, Hong et al. describe such patterning in Multiple Ink Nanolithography: “Toward a Multiple-Pen Nano-Plotter”, Science 286, 523 (1999), which is also incorporated by reference herein for all purposes. Interactions of immobilized proteins with ligands, antibodies or other molecules fundamentally change many of the physical properties of features at this small length scale, such as shape and size, which allows for the development of new screening strategies based on standard AFM imaging.
Another application that calls for precise alignment or registration of a probe tip with the surface of a substrate, is the fabrication of large scale sensing and processing arrays, such as those proposed as a lab-on-a-chip. Again, fabrication of such an array of sensors or nanodevices to form chemical, molecular, or electrical circuits with highly accurate spatial patterning and highly specific functionality, requires frequent modification and periodic replacement of the AFM probe tip being used for writing. Therefore, it is necessary to rapidly, repeatedly, and reliably return the AFM probe(s) to the same location so that the sequential patterning steps can be performed with sufficient precision.
Therefore, improved techniques for spatial alignment or registration of nanoscale probes with surfaces are highly desirable.